Method of operating a copper smelting furnace

ABSTRACT

In a method of operating a copper smelting furnace, wherein a ferrous substance containing more than 80 wt. % metallic iron having a specific gravity of 3.0-8.0 and particle diameter of 0.3-15 mm is added to copper smelting slag containing Fe having an oxidation-reduction number of 3+ and to the Fe 3 O 4  in the intermediate layer, thereby deoxidizing the Fe 3 O 4  to FeO, the method reduces the Fe 3 O 4  within the slag layer and the Fe 3 O 4  generated in the intermediate layer between the slag layer and the matte layer. So that their viscosity is reduced and separation rate is increased, thus increasing the yield rate of useful metal, and the problems that originate in the intermediate layer are eliminated.

BACKGROUND OF THE INVENTION

[0001] This invention relates to technology for reducing the amount ofFe₃O₄ in slag having a specific gravity of about 3.5-4.0, and in theintermediate layer between the slag and matte, the intermediate layerhaving a specific gravity of 4.0-5.0, in the setting area of coppersmelting furnace.

[0002] Normally, in a copper smelting furnace, pulverized raw copperconcentrate and silica sand are blown into the reaction shaft of thefurnace along with auxiliary fuel and oxygen-enriched air, and oxidationtakes place either in a gaseous-solid state or a gaseous-liquid-solidstate. The product of oxidation consists of the matte, in which valuablemetals such as copper are condensed, and the slag, which is produced bythe slag-making reaction between FeO (produced when iron reacts withoxygen) and SiO₂. These are segregated by settling in a receptacle. Theslag layer, which has a lower specific gravity, settles at the upperportion of the receptacle, while the matte layer settles in the lowerportion.

[0003] During the reaction described above, oxygen-enriched air can beapplied to the raw copper concentrate at a proportion in excess of or ata proportion less than the desired level, thereby causing variations inthe reaction process. In the former case, the oxidation of iron withinthe raw material proceeds too rapidly, causing a portion of the Fe tooxidize excessively from FeO wherein Fe has an oxidation-reductionnumber of 2+ to Fe₃O₄ wherein the Fe has an oxidation-reduction numberof 3+. Because Fe₃O₄ has a high melting point, the increase in theproportion of Fe₃O₄ within the slag increases its viscosity.

[0004] In addition, Fe₃O₄ has a high specific gravity and forms a layerbeneath the slag layer which is fused to the slag. If the proportion ofFe₃O₄ is high enough, this layer becomes clearly distinguishable fromthe slag layer. Since this layer is situated in the middle of the slaglayer and the matte layer, it is known as the “intermediate layer.” Asstated before, an increase in the production of Fe₃O₄ as a result ofvariations in the reaction process leads to an increase in the thicknessof the intermediate layer, which interferes with the segregation ofvaluable metals drifting within the slag layer.

[0005] In addition, oxidized matter formed excessively during variationsin the reaction can turn into powdered dust, which can be pulled intothe exhaust gas and drawn into the gas exhaust openings, creatingaccretion, part of which can then be retained and sink to the bottom ofthe receptacle, creating a buildup that lessens the holding capacity ofthe receptacle.

[0006] Thus, as described above, production of Fe₃O₄ resulting fromvariations in the reaction can cause loss of valuable metal driftingwithin the slag layer and difficulties in closing the slag tap hole, aswell as affect the temperature of the slag and the matte and thequantity of the valuable metal in the matte layer, thus causingundesirable effects in later processes.

[0007] Hence, there was a need to find a method to deoxidize the Fe₃O₄within the slag and the intermediate layers to FeO, thus decreasing theviscosity of the slag and reducing the amount of Fe₃O₄ within theintermediate layer.

[0008] Previously, the Fe₃O₄ at the lower portion of the receptacle wasdeoxidized to FeO by introducing blocks of pig iron (ingots shaped 280mm L×80 mm W×50 mm H, 5 kgs in weight, specific gravity of 7.0 to 7.8)from the upper portion of the receptacle and allowing them to sink tothe bottom. However, with this method, the pig iron block does notremain in the slag and intermediate layers but sinks to the bottom ofthe receptacle, and thus is not effective in deoxidizing Fe₃O₄ in theselayers. The present invention is based upon a relation between specificgravity and grain size of material effective for the deoxidization ofFe₃O₄ such that the material remains within the slag and theintermediate layers, whereby the deoxidization of Fe₃O₄ is effected.

SUMMARY OF THE INVENTION

[0009] The present invention comprises a method of operating a coppersmelting furnace wherein a ferrous substance containing more than 80 wt.% metallic iron, having a specific gravity of 3.0-8.0 and particlediameter of 0.3-15.0 mm, is added to copper smelting slag. The ferroussubstance is added to the Fe₃O₄ in the intermediate layer, therebydeoxidizing the Fe₃O₄ to FeO. More specifically, the present inventioncomprises a method of operating a copper smelting furnace wherein theferrous substance specified above is added to the intermediate layergenerated between the slag and the matte so as to reduce saidintermediate layer.

[0010] By employing the present invention, it is possible to reduce theamount of Fe₃O₄ within the slag layer and intermediate layer through thesimple method of adding grain-shaped matter from above. This allowsvaluable metals, such as copper, gold and silver drifting within theslag to sink more rapidly, thereby increasing their recovery rate. Inaddition, various problems in the intermediate layer are reduced,thereby allowing for more efficient operation of the copper smeltingfurnace.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a side view of a flash furnace and a slag-cleaningfurnace.

[0012]FIG. 2 is a schematic diagram of a crucible test.

[0013]FIG. 3 is a graph of crucible test results.

[0014]FIGS. 4A and 4B are illustrations of the difference in measurementof the layers in the settler before and after the addition of pig irongrains.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention is described in detail below.

[0016] As an example of a copper smelting furnace, FIG. 1 is a side viewof an Outokumpu flash furnace and slag-cleaning furnace used at NipponMining and Metals Saganoseki Smelter & Refinery.

[0017] The flash furnace is comprised of three parts: a reaction shaft 1having a burner 9 in the middle of the ceiling, a settler 2, and anexhaust pipe. The slag extracted from the settler 2 is passed to theslag-cleaning furnace 4 through the launder 6, where it is kept warm byresistance heating using Soderberg electrodes 8. The settler 2 and theslag-cleaning furnace 4 both serve as receptacles, and the slag andmatte are segregated by their difference in specific gravity. In bothfurnaces the matte is drawn out through matte tap holes located at thelower portion of the furnace, and the slag is drawn out through slag tapholes located in the upper portion of the furnace. In addition to thefurnace described above, where the slag is further treated in aslag-cleaning furnace 4 after being treated in the flash furnace, thereare many types of copper smelting furnaces and methods which use them.Most of them are based on a combination of a reaction shaft where rawcopper concentrate is oxidized, and a settling receptacle where theproducts are allowed to settle and segregate into matte and slag, butthere are some types of smelting furnaces where the reaction takes placewithin the settling receptacle. The present invention applies to alltypes of copper smelting furnaces that employ a settling receptaclewherein matte and slag are segregated by differences in specificgravity.

[0018] In the case of the flash furnace, raw copper concentrate mixtureand oxygen-enriched air are blown into the burner 9, and fall throughthe reaction shaft 1 as the reaction proceeds, the raw copperconcentrate mixture, which contains sulfuric material, transforms intomatte, slag, and a portion of the exhaust gas by the time it reaches thebottom of the reaction shaft 1. A portion of the products of thisreaction are pulled into the flow of the exhaust gas and fly toward theexhaust opening, and is known as “dust.”

[0019] The matte and slag that form within the reaction shaft 1 aresegregated by differences in specific gravity within the settler 2. Theslag is drawn out through the slag launder 6 of the flash furnace and isfurther divided into slag and matte in the slag-cleaning furnace 2. Thisslag is then drawn out through the slag-cleaning furnace's slag launder7. For reference, the specific gravity of the matte is 5.0-5.5, the slagis 3.6-4.0, and the intermediate layer is 4.0-5.0.

[0020] Deoxidizing agents having the specific gravity and grain size tobe retained within the slag and intermediate layers, namely a ferroussubstance containing more than 80 wt. % metallic iron, having a specificgravity of 3.0-8.0 and particle diameters of 0.3-15 mm, or, to specifythe composition in more detail, a ferrous substance containing Fe at90-97 wt. % and C at 3-6 wt. %, having a specific gravity of 3.0-8.0 andparticle diameters of 0.3-15 mm, such as pig iron, is added from abovethe slag. The word particles, as used in this specification, refers toboth particles and grains of particulate matter. Ferrous substancescontaining 60-80 wt. % metallic iron are also effective in deoxidizingthe Fe₃O₄ to FeO, though the rate of deoxidization per kilogram isreduced. Openings 5 for adding deoxidizing agents are mounted at variouspoints in the settler 2 and the slag-cleaning furnace 4, and areadjusted according to the conditions of the slag layer and theintermediate layer.

[0021] It is desired that the ferrous substance have a specific gravityof about 3.0-8.0. If the specific gravity is less than 3.0, thesubstance does not satisfactorily reach the intermediate layer, thusonly deoxidizing the Fe₃O₄ within the slag layer, which is not desired.If the specific gravity is greater than 8.0, the substance penetrates tothe matte layer or to the bottom of the furnace, promoting the erosionof the bricks at the bottom of the furnace, which is not desired. Thegrain size of 0.3-15.0 mm again allows the ferrous substance to beretained within the slag layer and reach the intermediate layer, anddeoxidizes the Fe₃O₄ within the slag and the intermediate layers withoutreaching the matte layer. This deoxidization reaction reduces the amountof Fe₃O₄ within the slag layer and the intermediate layer, thus loweringthe viscosity of the slag layer and reducing the intermediate layer.

[0022] The following is a list of the items identified by referencenumerals in the Figures provided.

[0023]1. reaction shaft

[0024]2. settler

[0025]3. uptake

[0026]4. slag-cleaning furnace

[0027]5. opening for inserting deoxidizing agent

[0028]6. slag launder

[0029]7. slag launder

[0030]8. Soderberg electrodes

[0031]9. concentrate burner

[0032]10. opening for inserting raw material

[0033]11. opening for blowing in oxygen-enriched air

[0034]12. slag

[0035]13. crucible

[0036]14. outside crucible

[0037]15. thermoelectric thermometer

[0038]16. tube for blowing in nitrogen

[0039]17. lid of crucible

[0040]18. chute for inserting deoxidizing agent

[0041]19. bricks for adjusting position

[0042]20. Siliconit furnace

EXAMPLE 1

[0043] As an example of an application of the present invention, werelate an experiment performed by melting slag containing Fe₃O₄ in acrucible 13, and adding pig iron particles to its surface. Thisexperiment was performed using equipment as described in FIG. 2. 800 gof slag 12 were placed within the crucible 13 and the slag 12 was meltedwithin a nitrogen atmosphere simulating the inside of a flash furnace.Once the temperature reached 1270° C., the temperature was maintainedfor thirty minutes, after which 16 g of grains of pig iron (specificgravity 5.0-7.0) were added, and samples were taken periodically fromthe middle portion of the crucible to measure the deoxidization rate.The slag 12 within the crucible 13 was not stirred at all after theaddition of pig iron, and was maintained at a temperature of 1270° C.for 60 minutes. This experiment was repeated with different sized grainsof pig iron. As typical examples, FIG. 3 shows the results of two teststhat were conducted, one with grain particles under 1 mm and one withgrain particles between 1.00-3.36 nun. In both cases, the amount ofFe₃O₄ within the slag showed a reduction of 70-80 wt. % 20 minutes afterthe addition, clearly demonstrating the deoxidization effects of pigiron particles. The effects were more pronounced with particles withgrain size under 1 mm. Also, iron shot having a 1 mm diameter showed thesame effects as pig iron particles having a 1 mm-3.36 mm diameter.

EXAMPLE 2

[0044] Next, tests were conducted to confirm the deoxidization effectswithin an actual furnace. In this test, 50 kg of pig iron particles wereadded to the upper surface of the slag layer from a measuring hole (notshown) in the roof of the settler 2, positioned in the center of thesettler 2, relative to the direction of the slag flow.

[0045] The matte layer, intermediate layer, and slag layer weredistinguished by inserting a steel measuring rod having a diameter of 30mm and longer than the required length from the top of the settler 2into the metal slag inside the settler 2, then withdrawing it after aspecified time. The various layers are distinguished by observing thematerials adhering to the measuring rod. This is a widely-used measuringmethod that has been used for a long time in distinguishing slag andmatte layers within a copper furnace.

[0046] Changes in the materials adhering to the measuring rod are shownin FIG. 4A and 4B. The intermediate layer, which has high viscosity,adheres thickly to the measuring rod, creating an uneven surfacecontaining matte and half-melted matter. The matte layer, on the otherhand, flows easily and only a thin deposit thereof adheres to themeasuring rod and it has a smooth surface. A relatively thick deposit ofthe slag layer adheres to the rod, but the surface is smooth.

[0047] Two tests were conducted involving the adding of pig ironparticles, and as shown in FIGS. 4A and 4B the intermediate layer was200 mm and 170 mm respectively before the addition of pig ironparticles. Fifteen to twenty minutes later, the intermediate layer hadbeen respectively reduced to 100 mm and 80 mm, or by approximately half,and what had been the upper portions of the intermediate layer hadbecome distinguishable from the slag layer, thus clearly demonstratingthe reduction of the intermediate layer.

Comparative Example 3

[0048] As shown in FIG. 3, ferro silicon containing 8.5 wt. % Si havinga grain size under 3 mm showed little deoxidization effect, perhapsbecause the specific gravity, at 1.8, is low.

Comparative Example 4

[0049] With the prior method of adding pig iron blocks, effects such asthose described above are not obtained, since the pig iron blocks arenot retained within the slag layer and the intermediate layer.

[0050] By employing the present invention, it is possible to reduce theamount of Fe₃O₄ within the slag layer and intermediate layer through thesimple method of adding grain-shaped matter from above. This allowsvaluable metals, such as copper, gold and silver drifting within theslag to sink more rapidly, thereby increasing their recovery rate. Inaddition, various problems induced by the presence of the intermediatelayer are reduced, allowing for more efficient operation of the coppersmelting furnace.

What is claimed is:
 1. A method of operating a copper smelting furnace,comprising adding a ferrous substance containing more than 80 wt. %metallic iron having a specific gravity of 3.0-8.0 and a particlediameter of 0.3-15.0 mm to copper smelting slag containing Fe having anoxidation-reduction number of 3+ and also to Fe₃O₄ in an intermediatelayer, thereby deoxidizing the Fe₃O₄ to FeO.
 2. A method of operating acopper smelting furnace according to claim 1 , comprising adding ferroussubstance according to claim 1 to an intermediate layer which is locatedbetween the slag and matte layer, thereby reducing an intermediatelayer.